In many radio communications systems it is desirable to maintain continuous bi-directional data transfer (full duplex operation) between two stations. Cellular telephone systems and wireless computer networking systems are examples of two such systems. Currently, in these applications, maintaining the full duplex mode of operation requires that the telephone or radio modem transmit on one frequency range (or band) and receive on another frequency range. This technique is termed frequency diversity. For instance, a cellular telephone may operate in a frequency range around a nominal 800 MHz. That range may extend from about 790 MHz to 810 MHz. The particular telephone may transmit in the lower region of the 800 MHz frequency range (for example 792 MHz to 798) while simultaneously receiving in the upper region of the 800 MHz frequency range (for example 802 MHz to 808 MHz). The frequencies used are usually separated by adequate guard-band (in this example 798 MHz to 802 MHz) so that frequency-selective filters can be used to isolate the transmitter from the receiver while at the same time coupling both the transmitter and receiver to a common antenna. This approach is also known as frequency diplexing. Other techniques, such as the use of circulators, time diversity techniques, spread spectrum codes, or polarization selectivity, have also been employed to separate the transmit signals from the receive signals for full duplex operation, over a single antenna.
During full duplex operation it is crucial that the desired signal from the antenna that appears at the receiver input be stronger than the leakage signal from the transmitter (at the receiving frequency) that appears at the receiver input. For a typical 1-watt (+30 dBm) transmitter, and a received signal strength of −70 dBm at the antenna, the transmitter power at the receiver's frequency must be suppressed by at least 100 dB at the input to the receiver. This is usually achieved by requiring that transmitters have strict limitations on out-of-band emissions, by receiving in a frequency band isolated and separate from that of the transmitter, and by employing high gain antennas to boost the received signal power. If the transmitter power is not suppressed sufficiently at the receiver input, then the sensitivity of the receiver is deteriorated, even though operation may still be possible at some impractically high receive signal levels. Power levels at the receiver input from communication signals captured by the antenna are often in the range of −90 to −20 dBm, so insufficient suppression of the transmitter output will limit the useful range of the receiver and the distance over which full duplex radio communication may be established.
In military radios, due to the spread-spectrum coding and modulation schemes, the signals are spread over several octaves of bandwidth and are at power levels reaching hundreds of watts in CW. For example, the military SINCGARDS radios operate in the 30-88 MHz range at a maximum output power of 50 W per radio. In a cluster of 4 radios, operating simultaneously on a vehicular platform, there exists a worst-case scenario, in which 1 radio is receiving and 3 radios are transmitting, that produces 150 W of transmitting power to interfere with the receiving radio. The issues of co-site interference here are prevalent and enormous. A solution to these co-site issues is the quasi-circulator.
Circulators are known in the industry and provide a means of coupling both a transmitter and a receiver to a common antenna. A circulator is a three-port ferrite (magnetic) device that operates over some RF (radio frequency) bandwidth, and is illustrated schematically in FIG. 1. A circulator preferentially and circularly transfers power from Port 1 to Port 2, from Port 2 to Port 3, and from Port 3 to Port 1, hence the name. Power input to Port 1 of a circulator will appear mostly at Ports 2 and very little at Port 3. Typically, about 20 dB less of input power appears at Port 3. FIG. 2 shows Circulator 14 used to isolate a transmitter from a receiver, and to couple both to a common antenna. In this instance, the circulator provides 20 dB of isolation between the transmitter and receiver. 20 dB of isolation is usually insufficient to prevent power from the transmitter from interfering with a desired signal received from the antenna, so bandpass filters 15 and 16 are added to the transmit and receive signal paths, and frequencies of operation are chosen such that the transmitter signal passes through bandpass filter 15, but is blocked by bandpass filter 16, which in turn only passes the received signal from the Antenna 3. The use of bandpass filters 15 and 16 can suppress the transmitter power that enters the receiver by another 40 dB. This improves the isolation between transmitter and receiver to 60 dB, which is often enough to allow simultaneous transmission and reception of signals. This prior art implementation requires the use of widely separated frequencies for transmit and receive, to take advantage of the isolation provided by bandpass filtering. However, magnetic circulators are not available at all frequency ranges, especially at VLF, LF, HF, VHF and UHF band, and even if available they do not cover a wide bandwidth and can not handle high power.
Antenna polarization selectivity can be used to provide isolation between transmitter and receiver in a full duplex radio, but similar to the circulator approach described above, polarization selectivity usually provides only about 20 dB of isolation between the transmitter and the receiver. Systems which use polarization selectivity to isolate the transmitter and receiver usually also separate the frequencies of operation and employ band pass filtering on the transmitter output and receiver input to provide additional isolation.
FIG. 3 shows the circuit schematic of a Wilkinson divider. These dividers are sometime called “splitters”. Radio power dividers of this type were described in a 1959 paper by Ernest J. Wilkinson. FIG. 3 shows features of a 3-port Wilkinson divider available from suppliers such as Werlatone with offices in Brewster N.Y. These devices can be used as a power splitters as well as power combiners.
Prior art patents describing techniques for providing isolation include U.S. Pat. No. 4,051,475, Radio Receiver Isolation System issued to Campbell; U.S. Pat. No. 4,174,506, Three-port lumped-element circulator comprising bypass conductor issued to Ogawa; and U.S. Pat. No. 4,704,588, Microstrip Circulator with Ferrite and Resonator in Printed Circuit Laminate issued to Kane. No prior art has been shown to adequately address co-site interference mitigation for a system in which multiple like radios are operated in a multi-octave band at very high power levels.
What is needed is a better system for providing radio isolation, in scenarios within which the problem of co-site interference is highly prevalent and harmful.